Temperature compensation for ground piercing metal detector

ABSTRACT

A hand tool has a ground-piercing probe, which is manually insertable in the soil. The probe has a chamber in which an inductor is positioned and connected to metal detection circuitry. A pulse generator applies current pulses to an inductor for inducing eddy currents in a buried target object. After the pulse terminates, the decaying coil voltage is sampled at different times to detect both the presence and range of the object, as well as the type of metal in the object. The coil voltage samples are applied to signaling circuitry, which provides an audible signal which is a series of bursts of an audio tone. The pitch of the audio tone signals the presence, range and bearing of the buried metal object, while the pulse rate of the bursts signals the metal type. Signal variations resulting from temperature changes when the probe is inserted into soil are compensated for by detecting a signal proportional to the coil current during the pulse and subtracting a scaled portion of that signal from voltage samples.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation in part of application Ser. No.09/366,805 filed Aug. 4, 1999 now U.S. Pat. No. 6,326,790 and entitledGround Piercing Metal Detector Having Range, Bearing and Metal-TypeDiscrimination.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

(Not Applicable)

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to metal detectors for detecting buriedmetal objects and more particularly relates to a metal detector using aground piercing probe and associated electronic circuitry and methodsfor detecting not only the range of buried metal objects, but also thebearing and metal type of the buried metal object.

2. Description of the Related Art

Metal detectors have long been used by hobbyists as a favorable form ofrecreation, which offers not only an enjoyable activity, but also theopportunity to discover valuable and/or historical metal target objectsburied in the soil.

In conventional prior art metal detecting, a user laterally reciprocatesa metal detector, usually including a coil or other inductor, above asoil surface in a scanning pattern seeking an audible signal whichindicates the presence, under the soil surface, of a metal object. Thesoil in which such buried metal objects are sought includes not onlydirt and sand, and other manually movable or deformable earth surfacematerials, which are readily accessible to humans, but also includesunderwater lake and sea bottoms.

A commonly used signal which indicates the presence of a metal object isan increase in the frequency of an audible tone. The typical userreciprocates the inductor of the typical metal detector above thesurface in repetitive left and right, arcuate reciprocating movementsuntil the frequency increase is heard or a tone is heard. Upon hearing afrequency increase, the user then begins reciprocating the inductor insmaller arcs across the region where the increased frequency signal wasdetected in an attempt to more precisely determine a position verticallyabove the buried metal object.

When permitted, a shovel is then used to dig up a clump of soil and theuser then breaks apart and sifts through the clump using the user'sfingers or a tool in an attempt to find the metal target object. If theuser is fortunate, the target object will be found in the clump of soilwhich has been dug up. Unfortunately, conventional metal detectors oftencannot sufficiently accurately pinpoint the location of the buried metalobject. Therefore, the metal detector is again employed and additionalclumps of soil must be similarly dug up, sifted through and inspected.Also, the buried metal object may be located below the depth of theinitial dig, necessitating deeper digging to retrieve the objects.Consequently, conventional metal detecting typically requires extensivemanual labor for removing clumps of soil, breaking them apart andsifting through them.

After completing these activities, a conscientious metal detector userwill then carefully replace the soil in an attempt to return the soil asnearly as possible to its original condition. However, some users do notexercise such care, merely leaving a hole in the soil. Even users whocarefully replace the soil have nonetheless created and left behind asubstantial environmental disturbance in the soil.

These soil disturbances, especially at popular historical sites andhighly trafficked outdoor public or park areas, often cumulatively causeboth significant damage to the visual, cosmetic appearance of such areasas well as destruction of vegetation or other components of the localecosystem and the creation of safety hazards.

As a result, many owners of private land and operators of public parkshave imposed digging restrictions and regulations to minimize suchdamage or destruction. Typically, these restrictions limit the metaldetector operator to digging in the soil with a tool no larger than ascrewdriver and to digging only small holes to retrieve a metal targetobject. Because conventional metal detectors are insufficiently accurateto allow the target metal object to be located within a distance rangetolerance of such a small hole, the operators use a probe, such as anice pick or a screwdriver, to pierce the ground in the vicinity of thedetected object in an attempt to strike the object and feel itspresence. Such ground piercing does not cause significant soildisturbance and often is beneficial in providing aeration for trees orother vegetation. However, one problem with this technique is that it isa trial and error process which usually requires numerous groundpiercings because such a probe only has a detection width equal to itsown very narrow width. An additional problem is that such a probe andpierce technique does not enable the user to distinguish between thedetected metal object and a nearby stone, tree root or other buriedobject which is harder than the surrounding soil. Consequently, smalldigs resulting in retrieval of only a stone or exposure or damage to atree root, often occur. Furthermore, if an initial small hole has beendug and the object is not found, the problem remains for the user todetermine in which direction from the hole the buried target objectlies.

In an attempt to overcome the inherent inaccuracy of the conventionalmetal detector, the prior art has provided miniaturized metal detectors.These are of the same type as and modeled after the larger conventionalmetal detectors, but are much smaller. They are typically larger than ½″in diameter, have a plastic outer sleeve or shaft and use conventionalmetal detection circuitry. These miniature versions of conventionalmetal detectors are used to search for buried metal objects near thewalls of a hole the user has dug or in dirt the user has dug out to formthe hole and is sifting through.

There is therefore a need for a hand tool and associated electroniccircuitry and methods to more precisely locate the buried metal targetobject after the general vicinity of its location has been detected by aconventional metal detector.

An object and feature of the present invention is that, after thegeneral vicinity of a buried metal target object has been located by aconventional metal detector and before any holes have been dug,embodiments of the invention permit the location of the object to bemore precisely detected, utilizing considerably fewer soil piercingswithout significant disturbance of the soil.

Another object and feature of the invention is to provide a metaldetecting hand tool having a probe with sufficient strength, hardnessand toughness so that it can survive repetitive insertions intoundisturbed soil, especially hard soils and abrasive soils, such assand.

A further object and feature of the invention is that each time the soilis pierced by the probe of the invention, the bearing of the object maybe detected to guide the user toward the next most appropriate place toagain pierce the ground, thus greatly improving the probability that themetal object will be struck by the next ground piercing.

A further object and feature of the invention is to detect not only therange and bearing to the buried metal object, but to detect informationabout the type of metal in that object. This permits the user todiscontinue the effort to retrieve the metal object if the user is notinterested in objects of the detected metal type and also permits theuser to spend more time being more persistent if a potentially valuablemetal is detected.

BRIEF SUMMARY OF THE INVENTION

The invention is an improved hand tool for detecting buried metalobjects. The hand tool has a handgrip attached to a ground-piercingprobe, which is manually insertable in the soil to displace the soilradially outwardly from the probe. An improvement of the invention is achamber in the probe and at least one inductor positioned in thatchamber and connected to metal detection circuitry. The inventionincludes other features, such as the provision of an asymmetricallyshaped magnetic field pattern about the probe for providing directionalsensitivity and the use of particular materials in the probe.

The invention further is directed to a method for detecting the locationof a buried metal target object. The method is initiated by plunging theprobe of a tool embodying the invention a distance into the soil todisplace the soil outwardly from the probe and position the chamberbelow the surface of the soil. Eddy currents are then induced in thetarget object by generating a time changing magnetic field about thecoil located in the chamber. Current induced in the coil by the eddycurrents is then detected to determine the presence and/or range of themetal object and, if certain additional features of the invention areutilized, the bearing and metal type of the target object.

The invention further is a metal detection circuit for detecting thetype of metal in the buried metal object. The circuit provides a signalto the user which indicates whether the metal is a ferrous metal, suchas iron or steel, a high conductivity metal, such as pure gold, a silvercoin or a copper coin, or a medium conductivity metal, such as aluminumalloys or gold alloys commonly used for jewelry. The metal detectioncircuit has a pulse generator connected to a coil for generating a timechanging magnetic field in response to electrical pulses applied to thecoil. A resistive energy damper is coupled to the coil for attenuatingthe energy in the magnetic field in a manner which extends the decay ofthe current in the coil after termination of the pulse. A sampling andstoring circuit is connected to the coil for sampling a signalrepresenting the magnitude of the coil current at a sampling time.Preferably coil voltage is sampled in order to detect the coil currentat the sampling time because voltage and current are related by ohms lawand are therefore interchangeable signals. Consequently, mostqualitative observations made about coil current are also applicable tocoil voltage. The sampling time for detecting metal type is within atime interval beginning after termination of the pulse from the pulsegenerator and ending before the time at which the coil current in thepresence of a high conductivity metal target object decays to a currentwhich exceeds the current to which the coil current decays in theabsence of a metal target object. A signaling circuit is connected tothe output of the sampling and storing circuit for signaling changes inthe detected coil current as the coil is moved. In particular, thesignaling circuit signals whether the coil current increases, decreasesor remains substantially the same. The direction of change or the lackof change in the coil current as the coil is moved may then beinterpreted by the user, or a suitably programmed computer, as anindication of the type of metal in the buried metal object.

The invention further is a method for detecting the metal type of ametal target object buried below the surface of a soil. The methodbegins by attempting to induce eddy currents in a target object byapplying a current pulse to a coil which is coupled to a damping load.The current induced in a coil by the eddy currents is detected at afirst sampling time. The first sampling time is located within a timeinterval beginning after termination of the current pulse applied to thecoil. The time interval ends before the time at which the coil current,in the presence of a high conductivity metal object, decays to a currentwhich exceeds the current to which the coil decays in the absence of ametal target object. Changes in the detected coil current are monitoredand signaled as the probe is moved. These signaled changes in the coilcurrent sample magnitude as the coil is moved closer to the metal objectare then interpreted by the user to determine the type of metal objectburied beneath the soil.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a pictorial view illustrating the use of an embodiment of theinvention.

FIG. 2 is a view in vertical section of an embodiment of the invention.

FIG. 3 is a view in side elevation of an alternative embodiment of theinvention.

FIG. 4 is an enlarged view in side elevation of an inductor formed by acoil and ferrite core which has been contoured to provide anasymmetrical magnetic field.

FIG. 5 is a view in horizontal section through a probe of an alternativeembodiment for creating an asymmetrical magnetic field.

FIG. 6 is a view in horizontal section through a probe of an alternativeembodiment for creating an asymmetrical magnetic field.

FIG. 7 is a block diagram illustrating the fundamental principles of thepresent invention.

FIG. 8 is an oscillogram illustrating the principles of operation of thepresent invention.

FIG. 9 is a block diagram of the preferred circuit embodying the presentinvention.

FIGS. 10-13 are schematic diagrams of the preferred embodiment of theinvention, which, because of the size and detail illustrated, must bebroken among several sheets, but when connected together illustrate thepreferred circuit of the present invention.

FIG. 14 is a view in axial section taken substantially along the lines14-14 of FIG. 3.

FIG. 15 is a schematic diagram illustrating the preferred embodiment ofthe temperature compensation invention.

FIG. 16 is a detailed schematic diagram illustrating the detailedpreferred embodiment of the metal detector including the preferredtemperature compensation circuitry. FIG. 16 is divided among the fourFIGS. 16A, 16B, 16C and 16D, with indications of the circuit connectionsbetween the figures, in order to illustrate the details of the circuitin a manner meeting the drawing requirements.

FIG. 17 is a schematic diagram illustrating a constant currentembodiment of the temperature compensation circuitry embodiment

In describing the preferred embodiment of the invention which isillustrated in the drawings, specific terminology will be resorted tofor the sake of clarity. However, it is not intended that the inventionbe limited to the specific terms so selected and it is to be understoodthat each specific term includes all technical equivalents which operatein a similar manner to accomplish a similar purpose. For example, theword connected or terms similar thereto are often used. They are notlimited to direct connection but include connection through othercircuit elements where such connection is recognized as being equivalentby those skilled in the art. In addition, many circuits are illustratedwhich are of a type which perform well-known operations on electronicsignals. Those skilled in the art will recognize that there are many,and in the future may be additional, alternative circuits which arerecognized as equivalent because they provide the same operations on thesignals. Further, those skilled in the art will recognize that, underwell-known principles of Boolean logic, logic levels and logic functionsmay be inverted to obtain identical or equivalent results. Similarly,there are currently various materials existing, and are likely to beadditional materials developed in the future, which exhibit thecharacteristics described herein, making them useful in embodiments ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, the hand tool 10 of the present inventionhas a handgrip 12 attached to a ground-piercing probe 14. The probe 14extends perpendicularly from a central portion of the handgrip 12. Thehandgrip extends in opposite directions from an end of the probe toprovide a t-shaped hand tool, so that the handgrip can be comfortablygripped by the hand of a user 16 and manually thrust into the soil 18.When inserted in the soil 18, the probe 14 causes the soil 18 to bedisplaced a small distance radially outwardly from the probe 14. The end19 of the probe 14 is preferably rounded and most preferablyhemispherical to facilitate its insertion into the soil and to providestrength and durability.

The probe 14 is preferably tubular, so that it is provided with aninterior chamber 20. Alternatively, the chamber 20 may be formed as agroove, notch or cavity in the probe 14, but the tubular shape ispreferred because such a cavity within the tube provides both a chamberin which an inductor 22 may be mounted and through which a wire 24 mayeasily extend into connection with one terminal of the inductor 22. Aswill be described below, the probe 14 can be made of a variety ofmaterials and if S the probe 14 is a metal, the other terminal of theinductor 22 may be connected directly to the metal probe 14, which is inturn connected to a wire 26. The wires 24 and 26 connect the inductor 22to a metal detection circuit 28. If the probe 14 is a nonconductivematerial, the wire 26 also extends into the chamber into connection withthe other terminal of the inductor 22. Preferably, the wires 24 and 26extend from the interior, central portion of the handgrip 12, outthrough the end 30 of the handgrip 12 and they are sufficiently longthat the metal detection circuit 28 can be conveniently, releasablymounted to the belt, or other clothing article, on the user 16 orself-contained and hand-carried.

FIG. 3 illustrates some alternative features of a hand tool embodyingthe present invention. FIG. 3 shows a hand tool having a handgrip 32 anda probe 34 with length graduations 36 together with numerals indicatinglength dimensions spaced along the probe 34, with reference to therounded tip 38 of the probe 34. This allows a user to determine thedepth of an object which is struck by the probe. The user can thenchoose the most effective tool for extracting the target object. Thegraduations also provide an indication of the depth of an object whichis not struck, but is detected by the electronic circuitry in a mannersubsequently described as being located horizontally from the insertedprobe.

As will be described below, the tool can also have a directionalsensitivity in a plane which is perpendicular to the axis of the probe14 or 34, as a result of providing a radially asymmetric field patternaround the inductor 22. Consequently, it is also desirable to provide adirection pointing indicium, such as an arrow 40 on the handle 12,indicating the direction of maximum sensitivity, which is the directionof the major lobe of the field pattern. This indicium, of course, canalternatively consist of a dot or other geometric figure placed at oneend of the handle, a different coloration or other indicium to indicatethe direction of maximum radial sensitivity.

Certain dimensional features and material characteristics of theinvention are particularly desirable and preferred. The diameter of theprobe 14 or 34 should be less than about ⅜″ because it is too difficultor impossible to manually force the probe into many soils if it is anylarger. Preferably, a tubular probe has an outside diameter ofsubstantially {fraction (7/32)}″ or 0.226″, which are equivalent. Thepreferred probe 34 has two component parts illustrated in FIG. 3 and inmore detail in FIG. 14. One component is a #304 stainless steel tube 35having an outside diameter of 0.226″, an inside diameter of 0.080″ and alength of 12.5″. The stainless tube 35 is telescopically interfit intoand bonded to a second tip component along an annular shoulder segment37 which is 0.50″ in axial length. The tip component is a tubularzirconia tip 39 which has a cylindrical portion 1.5″ long with anoutside diameter of {fraction (7/32)}″ and an inside diameter of 0.145″and a hemispherical tip to provide an overall probe length of 14″. Alonger zirconia tip is weaker and a shorter zirconia tip isinsufficiently long to both contain a coil and fit over the stainlesstube. Other materials would also make desirable tips. For example, thetip may be constructed of a metal oxide, such as sapphire, or a glass orceramic.

Most conventional metal detection circuits, as well as the preferredmetal detection circuit of the present invention, include a headsetconnected by a wire to the main circuitry of the metal detection circuitto enable the user to hear an audio signal generated by the metaldetection circuitry. Sometimes audio speakers are also provided foralternative use. Alternatively, however, an FM transmitter 42 may beconnected in the principal metal detection circuitry 44 for modulatingthe audio signal generated by the metal detection circuitry 44 andtransmitting that signal to an FM receiver 46, mounted to the headset 48and electrically connected to its sound transducers. Of course, othertypes of modulation may be used.

As a further alternative, the metal detection circuitry may beminiaturized and mounted within the handgrip 12 or 32 of the presentinvention, including the FM transmitter, to entirely eliminate the needfor a wire extending out of the handgrip 12 or 32 to a remotely mountedmetal detection circuit.

Popular, prior art metal detection circuits utilize an inductor, usuallyin the form of a coil. The hand tool of the present inventionillustrated in FIGS. 1-3, can be used with these prior art metaldetection circuits with the appropriate selection of probe materials andthe coil. Preferably, however, the probe is used with the modified pulseinduction detection circuitry, illustrated in the Figures and describedbelow.

The prior art circuits, as well as the circuit of the present invention,detect the presence of a buried metal object by inducing eddy currentsin a buried metal object by means of a time varying current in a coil.The frequency of the coil current is sufficiently high to provideacceptable coupling and a sufficiently low frequency to provide adequatesoil penetration. The typical frequency of operation is 5 KHz.

One type of prior art metal detection system uses detuning of anoscillator as a result of inductive coupling between an inductor and aburied metal object. One embodiment of this system has a pair ofoscillators, referred to as beat frequency oscillators. One oscillatoris operated at a fixed frequency. The other oscillator is tuned tosubstantially the same frequency, but includes a coil, which is thesearch coil. In embodiments of the invention, the coil 22 in FIG. 2 maybe used as such a search coil. The outputs of these two oscillators aremixed to detect a difference audio frequency which provides an output,audio tone. As the search coil is brought into proximity with a buriedmetal object, the second oscillator is detuned in proportion to thesearch coil's coupling to the buried metal object. Consequently, changesin the pitch of the audio output tone signal the presence and range ofthe buried metal object. As the range decreases, the mutual inductivecoupling is increased causing the oscillator to be further detuned andtherefore raising the audio pitch.

Another variation of the oscillator detuning principle utilizes a singleoscillator connected to the coil, such as the inductor 22. Theoscillator has a conventional audio frequency control negative feedbackloop, having a signal which varies in magnitude in proportion todetuning of the oscillator by coupling of the coil to a buried metalobject. The magnitude variations of the AFC loop are used to control avoltage controlled oscillator, oscillating at an audio frequency, withthe result that the frequency of the audio output tone is varied as thecoil is brought into proximity with the buried metal object.

The coil 22 in embodiments of the invention may be used as the sensingcoil with both of these circuits embodying the detuning principle,although this is not preferred.

A second type of prior art metal detection circuit uses the inductionbalance principle. With such a circuit, two coils are orthogonallypositioned to provide a null condition; that is so that there is noelectromagnetic coupling between the coils. One coil transmits to theburied object at an oscillator frequency, while the other is used forreceiving a signal from the eddy currents induced in the object. When nometal object is present and consequently there is no metal object inwhich eddy currents are generated, little or no signal is received bythe receiving coil. However, the presence of a metal object in proximityto the coils allows the eddy currents generated in the metal object tobe transmitted back to and received by the receiving coil and detectedby the metal detection circuitry. The magnitude of this received signal,coupled from the transmitting coil to the metal object and then from themetal object back to the receiving coil, is applied to a signalingcircuit, typically an audio oscillator, to vary the audio tone inproportion to the magnitude of the received signal. This magnitudeindicates the presence and range of the buried metal object. The coilsof an induction balance detection system may also be mounted in theprobe 14 or 34 using the present invention, although this is notpreferred.

Both of the above described prior art metal detection systems, when theyare utilized with the structure of the present invention, require thatthe probe be formed of a non-metallic or dielectric material so thateddy currents are not generated in the probe itself. Suitable materialsinclude a relatively hard, durable plastic material, a composite such asa carbon fiber/epoxy composite, or a ceramic material, such as zirconia.The probe can also be constructed with multiple sections of differentmaterial, such as metal tube having a zirconia or other ceramic tipportion 39, illustrated in FIG. 3. This ceramic tip construction is alsoparticularly desirable in preferred embodiments of the invention.

The third type of metal detection circuitry known in the prior art usespulse induction detection. That system ordinarily utilizes a single coilfor both transmitting and receiving although some embodiments utilizetwo coils, one for transmitting and a different one for receiving. Inthe pulse induction detection circuitry, a pulse is applied to a coilwith a sharp cutoff to provide a step function which, as known to thoseskilled in the art, produces high frequencies. Thus, by pulsing thecoil, a time changing magnetic field is generated about the coil, whichinduces eddy currents in a metal target object. These eddy currents arethen coupled back to either the same coil or a second receiving coil andthe received signal is detected and applied to a signaling circuit.Since the magnitude of the received signal is a function of the distanceto a buried metal object, the signaling circuit signals changes inreceived signal magnitude to indicate the presence and range of themetal target. The present invention with the coil 22, illustrated inFIG. 2, may be utilized with prior art pulse induction detectioncircuitry. A second coil may be mounted in the probe 14 for use withpulse induction circuits utilizing two coils.

FIG. 7 is a simplified block circuit diagram illustrating thefundamental principles of the present invention and FIG. 8 illustratesthe operation of that circuit. FIG. 7 uses the pulse induction systemand therefore has a pulse generator 50, which applies periodic pulses toinductor 52. The term “inductor” is used to indicate an electroniccomponent which exhibits an inductance. The term includes a coil with nocore material as well as a coil with a core. As is known in the art,separate transmitting and receiving inductors may be used, but are notpreferred because the circuit is more simplified by elimination of oneinductor.

The pulses applied to the inductor 52 may, for example, be applied tothe inductor 52 at a 120 Hz rate, with a pulse width of, for example 150microseconds, for driving the inductor 52 at a current of, for example,1 amp. The choice of pulse rate results from an engineering tradeoffbetween an increase in power consumption and coil heating as frequencyis increased and a decrease in sensitivity as the frequency isdecreased.

A resistive energy damper 54 is coupled to the inductor 52 either bydirect conductive connection or by inductive coupling. The resistiveenergy damper 54 may be a resistor or a resistive network connected inshunt across the inductor 52. Alternatively, the energy damper may be anonferrous, high resistivity metal inductively coupled to the inductor52, such as a metal used to construct the probe of the presentinvention.

A sampling circuit 56 is connected to the inductor 52 for sampling thecoil voltage or current at selected sampling times subsequent to thetermination of the pulse from the pulse generator 50. Signalsrepresenting the magnitude of the samples are then applied to signalingcircuits 58 and 60.

As illustrated in FIG. 8, the pulse terminates at time T0, after whichthe voltage across the inductor 52 decays exponentially at a rate whichis dependent upon the resistive energy damper, the presence or absenceof a buried target object, its range and the type of metal of which thetarget object is constructed. FIG. 8 is a family of oscillograms ofexponentially decaying coil voltages for representative metals and forthe absence of a metal target. The metals represent ferrous metals, highconductivity metals and mid-range conductivity metals. This family ofoscillograms has the characteristic that there is an initial region ofexponential decay having a duration of several time constants duringwhich most of the decay of this family of curves occurs. This isfollowed by a region of slower decay rate.

FIG. 8 is a drawing of actual oscillograms. In the circuit from whichthese oscillograms were derived, the voltage pulse is clamped by a 15volt zener diode to protect the input of an op-amp from a 200 voltspike. Consequently, the coil current, when the coil voltage exceeds 15volts, is shunted through the zener diode. Because of the clampingdiode, during the initial, relatively flat voltage segment of theoscillogram, the coil current is exponential but the voltage is not.This early region of shunted coil current is not used by the circuit asa signal.

One important characteristic of this family of curves is that afterseveral time constants, the curve N, which represents the decay in theabsence of a metal target, decays to the lowest value of any of thedecaying curves. That is because there are no eddy currents induced in aburied metal object to store energy and couple energy back to the metaldetector circuit. In this region to the right of the knee of thesecurves, samples are taken to determine whether a metal object is presentand its range. For example, in the present invention samples are takenat time T4, which is preferably 34.1 microseconds after time T0, thetime of termination of the pulse applied to the inductor 52. The sampleduration is preferably 1 microsecond. Samples taken at this later timeinterval are applied to signaling circuit 60 for indicating targetpresence and range. As in prior art circuits, the magnitude of thesignals coupled from the eddy currents induced in the buried metalobject back to the receiving coil are an inverse function of the rangeto the metal object.

The reason that the inductor current takes longer to decay in thepresence of a metal object and therefore has a higher magnitude atsample time T4 is that energy is coupled to a metal object and generateseddy currents. These eddy currents generate magnetic fields in whichenergy is stored. Therefore, the pulse applied to the pulse generatorcauses more energy to be stored in the system in the presence of a metalobject, than in the absence of a metal object. Consequently, the decaytime is longer in the presence of a metal object.

At sampling time T4, although all decaying oscillograms which representmetal objects are greater than the oscillogram N representing theabsence of a target metal object, there is only a small differencebetween them. Consequently, samples taken in this later region aredifficult to use for distinguishing between the type of metal detected.However, since the magnitude of samples, taken beyond the knee of thecurve at time T4, vary inversely to the range of a metal object, samplestaken at this later time are used to determine the presence and range ofa buried metal object.

The signaling circuit 60 therefore signals to the user changes or lackof changes in the magnitude of the samples taken, for example at timeT4, as the inductor 52 is moved nearer or further from a buried metalobject. These changes may be signaled, for example, by varying the toneof an audio oscillator in proportion to the magnitude of those samples,or by other signaling means, such as a numerical display, a meter, alight which varies in intensity, or other signaling circuitry ordevices.

Another important characteristic of the decay oscillograms illustratedin FIG. 8 is the subject of the present invention. The rate of decay ofthe voltage across the inductor 52 during the first few time constantsof decay is also dependent upon the type of metal in the buried metalobject. At times preceding the knee of the oscillograms, the decayingvoltage for the different metals exhibit more substantial differences inmagnitude than after the knee. Therefore, samples can be taken at aselected time in this earlier portion of the decay and used todiscriminate between different kinds of metals.

In order to provide a time interval which is sufficiently long to allowsamples to be taken for detecting metal type, the time rate of decaymust be reduced (i.e. the decay time extended) from that in prior artpulse induction systems. In the prior art pulse induction systems, theinitial decay is so rapid that meaningful samples cannot be taken.

In the present invention, the decay rate is reduced (i.e. the decay timeand time constant are increased) by providing the resistive energydamper 54. Whether the resistive energy damper is a resistor, network ofresistors or other resistance, or is a nonferrous, low conductivitymetal forming the probe, the effect of the resistive energy damper is toreduce the decay rate and thus extend the time interval in which samplesmay be taken to determine the type of metal. The resistive energy damperreduces the rate of decay because the time constant for aresistive/inductive circuit is L/R. Consequently, the time constant isinversely proportional to the resistance of the energy damper, if theenergy damper is a resistance connected across the inductor 52.

The use of a nonferrous metal probe, instead of a resistance connectedacross the inductor, has a similar effect in reducing the rate of decayand thereby extending the time interval before the knee of the curves,during which samples can be taken for detecting metal type. However, thenon-ferrous metal probe should be a relatively high resistance (lowconductivity) metal to avoid extending the decay time of the eddycurrents in the metal probe beyond the time it takes for eddy currentsin a metal target object to decay. The extension of the decay timeoccurs because energy is stored in the magnetic field of the eddycurrents induced in the nonferrous metal probe material, prolonging thedecay time because more energy is stored in the coupled system. Thesystem can also be considered as analogous to a transformer in which thesecondary impedance reflected into the primary is inversely proportionalto the value of the secondary impedance. Therefore, the high secondaryresistance represented by the low conductivity metal reflects as a lowresistance into the coil to increase the circuit's time constant.

Another important characteristic of the decay oscillograms of FIG. 8 isthat the oscillogram I for iron and other ferrous materials has a decayrate which is substantially less than the decay rate N for no target.The reason ferrous metals extend the decay time is because of their highpermeability thus increasing the inductance of the coupled circuit. Theincreased inductance means more energy stored and therefore a longertime delay for dissipating that energy.

Yet another important characteristic of the decay oscillograms is thatthe decay oscillogram S for the high conductivity metals, such assilver, pure gold and copper, exhibits a substantially more rapid rateof decay than the decay rate for oscillogram N for no target. This maybe explained as the reflection of the low resistance of the highconductivity target into the coil as a high resistance, thus reducingthe time constant L/R. It may also be explained by observing that eddycurrents reflect a back emf into the coil, which opposes coil current.This mutual coupling reduces the apparent inductance of the coil.Because the eddy currents in a high conductivity metal take longer todissipate (because of the lower resistance) the back emf induced in thecoil by the eddy currents is greater for a longer time interval than formetals of lesser conductivity. High conductivity metals have an effectopposite the effect of ferrous metals because they reduce the apparentinductance of the coil mutually coupled with the metal object more thanmetals of less conductivity. Metals of intermediate conductivity, suchas 14 karat gold, an alloy commonly used in jewelry and plotted in FIG.8 as oscillogram G, or aluminum alloy decay at an intermediate ratebetween the decay rates for the high conductivity metals and the ferrousmetals.

Consequently, as the inductor 52 is moved from a position remote from aburied metal object, at which there is essentially no coupling to theobject, toward the buried metal object, samples taken in this earlyinterval, for example at time T1, will increase in magnitude if themetal is ferrous, decrease in magnitude if the metal is one of highconductivity, such as pure gold or silver, and not change much in thepresence of a jewelry gold metal object. Although the intermediateconductivity metal object sample magnitude will not change much as theobject is approached by the metal detector, the samples at time T4,which indicate range, will increase as that object is approached,thereby signaling that a metal object is being approached.

The signaling circuit 58 may be provided with a signal representing themagnitude of samples taken at time T1 and signals changes in thatsampling magnitude. Consequently, the user will be aware that a metalobject is present by a variation in the signal from the range signalingcircuit 60 and, knowing that, the user will be able to determine thenature of that metal object by the changes, or lack of changes, in thesignal from the metal type signaling circuit 58.

The time interval in which the sampling is done to detect the type ofmetal in the buried object may be seen with reference to FIG. 8. Afterseveral time constants, the decaying oscillograms for all metals aregreater than the decaying oscillogram for no target N. The decayoscillograms for high conductivity and medium conductivity metalsinitially follow a path to the left of the oscillogram N and eventuallyintersect it. The decay oscillogram for ferrous metals remains to theright of the oscillogram N. It is therefore necessary that samples takenfor the purpose of detecting the type of metal must be taken before thehigh conductivity metal oscillograms intersect the no target oscillogramN at time T5. Therefore, the sampling time for obtaining a sample todetect the nature of the metal must occur within a time intervalbeginning after termination of the pulse at time T0. That time intervalends before the time at which the inductor current in the presence of ahigh conductivity metal object decays to a current which exceeds thecurrent to which the inductor current decays in the absence of a metaltarget object; i.e. before time T5 in FIG. 8.

An ideal time position within that time interval to sample the coildecay voltage would appear to be time T1, which, in the embodimenttested, is approximately 7 microseconds after T0. The time T1 ispreferred because it is the time of intersection between the no targetoscillogram N and the oscillogram G for 14K gold. By utilizing thesampling time T1, samples for 14K gold would not vary in magnitude fromsamples taken for a no target condition and this lack of change wouldsignal to the user that the target being detected by the range sample attime T4 may be a 14K gold target. The exact time T1 at which the goldoscillogram G and the no target oscillogram N intersect is a function ofcircuit values, including the resistive energy damper 54 and theinductance of the inductor 52. Therefore, different sampling times willbe used with different specific circuits embodying the presentinvention.

Although the circuit operates well by taking a single sample at time T1within the time interval between T0 and T5, experimental usage of suchan embodiment of the invention has indicated that it is sometimesdifficult to precisely locate the sample time T1 at the exactintersection of oscillograms G and N. That fact, plus other circuit andenvironmental condition variables, sometimes makes it difficult todetermine whether the magnitude of the sample taken at time T1 isincreasing to indicate the presence of an iron target, decreasing toindicate the presence of a high conductivity target, or is changing onlyin a small amount as a result of the circuit variables to indicate thepresence of an intermediate conductivity metal, such as 14K gold.

As a consequence, the sampling method may be further improved by takingtwo samples at times T2 and T3 in the time interval between T0 and T5.The time position for this sampling at time T2 and T3 is slightly beforetime T1 and slightly after time T1 and in the present invention ispreferably at 5.85 microseconds following T0 and 8.3 microsecondsfollowing time T0. These two sample times are alternately selected bythe user by switching a manual switch between two positions.

Consequently, in operating the circuitry, the user will first use anembodiment of the invention with the metal-type selecting sample time ateither T2 or T3. Upon perceiving a signal indicating a variation insample magnitude for samples taken within the interval T0-T5, forexample at T2, the user mentally notes the direction of change. The userthen switches to the other sample time, for example T3, and compares thesignaled direction of change to the previously noted direction. If thedirections of change of the sample magnitude signaled to the user is thesame in both instances, for example an increase in both instances, theuser will know that a ferrous metal has been detected. Conversely, if adecrease in sample magnitude is signaled to the user for both samplestaken at time T2 and time T3, the user will know that a highconductivity metal, such as silver, is being detected. However, if thedirection of change is different for one sample time, for example T2,than it is for the other sample time, for example T3, the user will knowthat a metal of intermediate conductivity has been detected.

From the above discussion of the theory of operation of the invention,it can be seen that the invention presents a manner of measuring theeffect of a change in the apparent inductance of the coil inductor,resulting from mutual coupling with the metal object. The circuitdetects a signal, which is a function of the coil's apparent inductanceresulting from mutual coupling between the coil and the eddy currentscirculating in the metal object. The circuit then signals the directionand magnitude of change in the apparent inductance of the inductor asthe probe is moved. This is done by monitoring a signal, such as coilvoltage, which changes as a function of a change in apparent coilinductance.

The change in apparent coil inductance can be used to signal the type ofmetal for the following reasons. For buried metal objects of anyconductive material, eddy currents are induced in the object when theeddy currents and the coil are mutually coupled and the coil is excitedby a time changing current. The mutual coupling reduces the apparentinductance of the coil and does so as a function of the conductivity ofthe conductive material in the buried object. The higher theconductivity of the buried object, the more the apparent inductance ofthe coil is reduced.

With ferromagnetic materials although the same phenomenon is present,there is simultaneously also an additional phenomenon that theinductance is increased because of the increase in permeabilityresulting from the high permeability of ferromagnetic materials. Becausethe conductivity of ferromagnetic materials is relatively low, theeffect of an increased inductance caused by the permeability offerromagnetic materials is considerably greater than the effect of areduced inductance caused by the mutual coupling with a conductivematerial. Therefore, ferromagnetic materials exhibit a net increase inapparent inductance of the coil. However, in the absence offerromagnetic permeability, buried metal objects cause a decrease inapparent inductance and the magnitude of that decrease is indicative ofthe conductivity of the metal in the buried metal object. Consequently,the direction of change in apparent inductance of the coil together withthe magnitude of that change can be used to signal the type of metal.

The inductors used in most prior art metal detection equipment arecoils. The present invention includes the use of high permeability coresfor increasing the inductance of the inductor. The inductor preferred inthe present invention is a coil with a ferrite core, such as commonlyused as an unshielded RF choke on printed circuit boards. The axis ofthe inductor is preferably parallel to the axis of the probe. A distinctand advantageous feature of the present invention is to provide aradially asymmetric magnetic field pattern about the inductor andtherefore about the axis of the probe in order to have directionalsensitivity, which can be used to detect the bearing to the buriedobject.

FIG. 4 illustrates a coil 70 with a ferrite core 71 of the typedescribed above, but modified by beveling its ends 72 to provide anasymmetric shape to the core. This asymmetry slightly reduces themagnetic field in one radial direction with respect to a magnetic fieldmaximum in the diametrically opposite direction. The use as the searchcoil of a small, commercially available inductor of the type commonlysold for use as an RF choke or in resonant circuits provides a coilslightly greater than ⅛″ in diameter and ¾″ long, thus allowing theprobe diameter to be small enough for easy insertion into the soil.Although this size coil is preferred, a larger coil may be used, but thecoil is preferably no larger than ¼″ in diameter so that the probediameter will not exceed ⅜″ in diameter, as previously explained.

An alternative manner of attaining radial asymmetry of the magneticfield is illustrated in FIG. 5, in which a coil 74 is radially offsetfrom the center of the probe 76. Each of the inductors or coils may beheld in place within the chamber with a suitable epoxy.

FIG. 6 illustrates yet another alternative manner of accomplishing theradial asymmetry. In that embodiment, the coil 78 is centrallypositioned within the probe 80 but a ferromagnetic material, such as aniron bar 82 is also positioned in the probe 80 parallel to its axis. Theiron bar 82 provides a low reluctance flux path so that the magneticfield radially outward from the iron bar 82 is reduced. The result is afield pattern which is asymmetric about the central axis of the probehaving a major lobe 84 of maximum sensitivity extending radiallyoutwardly from the side of the probe 80 opposite the iron bar 82.

The radial asymmetry of the magnetic field about the axis of the probepermits rotation of the entire hand tool about the axis of the probe,after the probe has been inserted into the soil, in order to cause aresulting variation in the detected signals, such as the samplemagnitudes. When a buried metal object is detected, the probe is rotateduntil the maximum signal is received, the maximum signal correspondingto the apparent closest range to the object. The bearing from theinserted probe to the buried object is now aligned with the direction ofthe major lobe. That direction is marked by suitable indicia on thetool, as described above. The angular direction from the probe to thetarget is the bearing.

The asymmetric field pattern feature of the present invention, referredto above and described in more detail below, for providing directionalsensitivity can be applied to prior art metal detection circuitry, aswell as to circuitry of the present invention.

As stated above, an alternative manner of delaying or extending thedecay time of the coil current is to utilize, as the resistive damper, anonferrous, high resistivity material, in close proximity to a detectingcoil. “Nonferrous” means a metal having a very low permeability. For thepresent invention, the permeability should be under 1.5. Such amaterial, for example stainless steel or titanium, has the additionaladvantage that it makes a probe which is stronger and harder andtherefore more durable and long lasting as it is forced into hardgrounds under substantial bending forces and undergoes repetitiveabrasion in soil materials. Stainless steel #305 and grade 5 titaniumare most preferred.

Because such a metal is nonferromagnetic, it does not shield themagnetic field from the coil. A nonferromagnetic probe material storesenergy in the magnetic field generated by eddy currents induced in itand has a high resistivity permitting it to attenuate that energy, in amanner which extends the decay time of the oscillograms of FIG. 8 in thesame manner as accomplished by a shunt resistance. If a metal used toconstruct the probe were of medium or high conductivity (lowresistivity), it would extend the decay times (reduce the decay rate) bytoo much. Such a metal would extend the decay times beyond the time atwhich the eddy currents induced in the buried metal object decay to nearzero, thus preventing detection of the range of the buried metal object.

The resistivity of a metal used as a probe should be 40 microohms-cm orhigher and preferably more than 70 microohms-cm. For example, grade5-titanium alloy has a resistivity of 177 microohms-cm and works welland is stronger than stainless. Grade 9 has a resistivity of about 140microohms-cm.

FIG. 9 is a block diagram illustrating the preferred embodiment of theinvention. The pulse generator 89, enclosed by dashed lines, comprisesan astable, multivibrator 90, operating at 240 Hz, the output of whichis applied to a divide by two flip-flop 92 to generate a 120 Hz signal,which in turn is amplified by the amplifier 94 and applied to theinductor 96. The resistive damper can be either a shunt resistance 97 ora high resistance, nonferromagnetic probe material as described above.The circuit also has a sampling circuit 98, enclosed by dashed lines,and a signaling circuit 100, also enclosed by dashed lines.

The sampling circuit 98 includes a first timing circuit 102, in the formof a monostable multivibrator, having its input connected to receive 120Hz pulses from the flip-flop 92 of the pulse generator 89. The samplingcircuit also has a second timing circuit 104, also in the form of amonostable multivibrator, having its input connected to the 240 Hzoutput of the astable, multivibrator 90 of the pulse generator 89. Thefirst timing circuit 102 is connected to control a switch 106 forsampling the coil voltage within the above-described time interval forthe purpose of detecting the type of metal in a buried metal targetobject. These samples are stored on sample storage capacitor CS2.

Similarly, the second timing circuit 104 is connected to control asampling switch 108, for sampling the coil voltage well after theabove-described time interval for detecting samples from which thepresence of the buried metal object and its range can be detected.

The voltage across inductor 96 is amplified by a low-level amplifier110, sampled by switch 108 and the sample is applied through DC blockingcapacitor CB to an absolute value amplifier which is made up of a highgain amplifier 112 followed by op-amp 114.

As a result, a DC signal proportional to the range sample level sampledby switch 108 is applied through amplifiers 112 and 114 to a voltagecontrolled oscillator 116. The voltage controlled oscillator 116 isdesigned to generate an audio frequency at approximately 200 Hz for thelowest anticipated range sample magnitude and 3500 Hz for the highestanticipated range sample amplitude, with interposed range sampleamplitudes generating correspondingly interposed frequencies. Obviouslya different frequency range may easily be utilized.

The metal type samples stored on capacitor CS2 are applied throughamplifier 118 to the input of a second voltage controlled oscillator120. Amplifiers 114 and 118 have conventional threshold controls 122 and124 respectively for setting the sample level below which the output ofthe amplifiers 114 and 118 remain at their lowest levels so that thevoltage controlled oscillators 116 and 120 do not respond to them. onlysample magnitudes greater than the adjustably selected threshold valueswill cause an increase in the output frequency of the voltage controlledoscillators 116 and 120. This performs a squelch operation whicheliminates response of the signaling circuit to noise or othermeaningless low power signals.

The audio output from the voltage controlled oscillator 116 is appliedthrough a switch 126 to an audio amplifier 128 and a speaker 130.However, the switch 126 has its control input connected to the output ofthe voltage controlled oscillator 120, so that it is periodicallyswitched to alternately connect and disconnect the audio tone from thespeaker at the frequency of the voltage control oscillator 120.

In the operation of the circuit of FIG. 9, the astable, multivibrator 90applies 240 Hz pulses to the monostable, multivibrator 104 at 240 Hzand, through the divide by 2 flip-flop 92, applies 120 Hz pulses to themonostable, multivibrator timing circuit 102, as well as through theamplifier 94 to the inductor 96. The monostable, multivibrator timingcircuit 102 applies a pulse of 0.5 microseconds pulse width to theswitch 106, approximately 7 microseconds after termination of the 120 Hzpulse. This closes the switch 106 for 0.5 microseconds, applying avoltage sample to capacitor CS2, which is held on the capacitor CS2 fordetermining metal type when the switch 106 is opened. Similarly, themultivibrator timing circuit 104 applies a pulse of 5 microsecondduration to the switch 108 at approximately 34.1 microseconds after thetermination of the pulse applied to the inductor 96. This causes asample voltage to be applied through capacitor CB to the amplifier 112for determining range.

The range samples require significant amplification by the amplifier 110because they are taken after nearly all decay of the coil voltage hasoccurred. Because the amplifier 110 is a very high gain amplifier,having a gain on the order of 1,000, drift resulting from aging andtemperature changes in the associated electronic components can cause asignificant error in the sampling signals. Consequently, the switch 108is switched at a 240 Hz rate, with the result that it is closed, notonly during the desired sampling time at 34.1 microseconds followingtermination of the pulse, but also at a time before the next 120 Hzpulse is applied the inductor 96 and after the voltage on inductor 96has decayed to essentially zero. This allows a DC level corresponding tothe drift to accumulate on the capacitor CS1 so that only the magnitudeof the range sample itself is coupled through the blocking capacitor CBto the amplifier 112. Those range samples are applied to the absolutevalue amplifier, comprising amplifiers 112 and 114, as a rectangularwave centered on zero volts. This rectangular wave has an amplituderepresenting the range sample magnitude and is converted to a DC levelproportional to that range sample magnitude by the absolute valueamplifier.

As a result, the output 132 of the sampling circuit provides a DC levelcorresponding to the magnitude of samples taken within theabove-described time interval, the magnitude of which represents thetype of metal in the buried metal object. Similarly, the output 134 ofthe sampling circuit 90 has a DC level corresponding to the magnitude ofsamples taken after the above time interval and represents the presenceand range of a buried metal object.

Since the range samples at terminal 134 are applied to control thefrequency of voltage control oscillator 116, the tone of the frequencyemitted from the speaker 130 signals the presence and range of theburied metal object in a manner corresponding to the signal emitted fromconventional metal detectors. In particular, the frequency of the toneincreases as the sample magnitude increases to signal that the inductor96 is getting closer to a buried metal object.

However, this audio tone is switched on and off at a rate which is afunction of the magnitude of the metal type samples provided at output132 of the sampling circuit 98. This very low switching rate, preferablyin the range of 2-12 Hz, periodically interrupts the tone to send burstsof audio tone to the speaker 130. The user can hear that interruptionrate.

Therefore, the user, before utilizing the embodiment of the invention,first sets the threshold circuit 124 to an intermediate rate between 2and 12 Hz. As a metal object is approached, which is signaled by achange in the audible frequency from voltage controlled oscillator 116,the user can tell whether the low frequency switching rate from voltagecontrolled oscillator 120 increases, decreases or remains about thesame. If the switching rate of voltage controlled oscillator 120alternately switches the audio tone on and off at a reduced switchingrate, the user can recognize that a highly conductive metal, such aspure gold, silver or copper, is being approached because the samplemagnitude will be decreasing. Similarly, if the switching rate of thevoltage controlled oscillator 120 increases, the user can perceive thatthe metal object being approached is a ferrous metal because the samplemagnitude will be increasing.

If the alternate switching rate of the audio tone changes very little ornot at all, the user will know that the metal being approached is one ofintermediate conductivity, such as aluminum alloy or 14K gold.

For providing two alternatively user selectable metal type samplingtimes T2 and T3, as described above, monostable multivibrator 102 maysimply have two, alternatively selectable timing resistances connectedso either timing resistance can be switched into the circuit. This may,for example, be accomplished by using two series resistors, one of whichis shunted by a manually operable switch 135, in FIG. 9, so that theseries resistors exhibit one resistance when the switch is opened and alesser resistance when the switch is closed. This is also shown in thedetailed circuit of the preferred embodiment illustrated in FIGS. 10-13.

In the event a designer prefers to use a pulse induction system usingtwo coils, the connection between the output of amplifier 94 andinductor 96 is omitted and instead a transmitting coil 96A is connectedto the output of amplifier 94, as shown in phantom in FIG. 9. In thatcase, the inductor 96 becomes the receiving coil.

While the switches are illustrated by conventional schematic symbols,the term “switches” is used in its usual broad, electronic meaning andis not limited to mechanical switches. In fact, transistor switches orgates are preferred. Of course, as known to those skilled in the art,the switching function can be performed by computer software toaccomplish the equivalent result. Similarly, as is known to thoseskilled in the art, the switching and sampling functions, audio signalgeneration, sample magnitude comparison, frequency variation andintermittent burst switching, as well as the pulse generator andsampling timing circuits, can all be equivalently performed using asuitably programmed, digital computer or microprocessor.

Turning now to the operation of an embodiment of the invention, a personusing the present invention first uses a conventional metal detector tofind a buried metal target object and to determine its location asprecisely as is possible. An embodiment of the invention is then graspedaround the handgrip and the soil is pierced as the probe is inserteddown into the soil at that location. If the probe strikes a hard objectand the object is metal, variations in the signals, described above,will occur as the coil located in the chamber of the probe approachesthe buried object and will signal that the object is a metal target andthe type of metal, as described above. If the object struck is a stoneor tree root, the signals will not vary appreciably.

However, if the probe does not strike a hard object in the soil, theuser then alternately raises and lowers the probe within the soil and,if the metal object is near the probe, variations in the audio frequencyof the range signal can be noted. The user raises and lowers the probeseeking the maximum frequency and can then read the graduations todetermine the depth below the ground surface of the buried metal object.

After determining the depth of the buried metal object, the user thenrotates the tool of the present invention about the central axis of theprobe to obtain a maximum audio frequency. The indicia on the probeindicating the major lobe of sensitivity then will indicate thedirection the user should move the probe to again insert it into thesoil. The user can use conventional geometrical locating techniques,such as those used with radio direction finders, to plot or mark a lineof position for the first bearing. The probe may then be inserted asecond time and the process repeated to detect a second line ofposition, making it likely that the buried metal object is located inthe vicinity of the intersection of the two lines of position.

Additionally, the type of metal object can be detected, by notingwhether the rate of the audio bursts is changing and, if so, whether itis increasing or decreasing.

Therefore, it can be seen that embodiments of the invention reduce andminimize both the number of times the ground must be pierced by theprobe, as well minimizing the number of soil excavations to retrieve aburied metal object. The volume of soil which must be excavated is alsominimized since the tool of the present invention provides range andbearing information with each insertion into the soil. Because theinvention allows a user to determine the type of metal before the soilis disturbed, the user can exercise the option to not retrieve theobject, thus reducing effort and soil disturbance. The inventionenlarges the effective detection area sensed by the probe. Instead ofrelying solely upon physical contact which the operator can physicallyfeel, as with conventional, inactive probes, the present inventionprovides range and bearing information over a larger search area.Furthermore, the present invention allows the discrimination of threetypes of metals, ferromagnetic metals, nonferrous, high conductivitymetals and nonferrous, mid-range conductivity metals. These principlesfor detecting the type of metal can be applied to conventional metaldetectors which detect solely above ground. The preferred embodiment ofthe invention ultimately provides an audible signal which is a series ofbursts of an audio tone. The pitch of the audio tone indicates thepresence, range and bearing of a buried metal object, while the pulserate of the bursts indicates the metal type.

FIGS. 10-13 illustrate the details of the preferred embodiment of theinvention. These details are not necessary to understand the invention,but are included to illustrate the presently preferred mode forpracticing the invention. Many of the functional groups of electroniccomponents have been framed in boxes to facilitate explanation of theoperation of the circuitry. The circuit includes many additionalfeatures which do not form a part of the invention, but are included inthe preferred embodiment of the invention to enhance its operation.

FIGS. 10-13 illustrate a single circuit with the figures being connectedtogether at the connector symbols, indicated at the periphery of thecircuit. Consequently, a description of the preferred embodiment ofFIGS. 10-13 will be given with simultaneous reference to all of thesefigures. The corners of the assembled schematic have dual letters, suchas UL to signify upper left, to assist in their arrangement.

The circuit is powered by battery BAT1 or a spare battery BAT2, which isalternatively selectable by double-pull, double-throw switch SW2. Thepower is switched on and off by switch SW3C. Block 201 provides a lowbattery alarm, so that when the negative power supply falls under itsregulated voltage the LED D1 mounted on a control panel is turned on tosignal a low battery.

Block 202 is the monostable, multivibrator timing circuit forcontrolling the sample timing and sample width for obtaining themetal-type identification sample. It corresponds to timing circuit 102of FIG. 9.

Switch SW1, in conjunction with monostable, multivibrator timingresistors R14 and R15, permit the user to alternatively select the twometal-type identification sample times described above to assist theuser in identifying the presence of middle conductivity metals.

In block 203, potentiometer R11 is the range, threshold pitch controland potentiometer R9 is the metal-type identification pulse rate orburst rate threshold control. The other components in block 203 adjustthe sensitivity and tracking for these controls.

Block 204 has an integrated circuit regulator U5 for regulating thepositive power supply voltage to 5 volts.

In block 205, the metal-type identification sample signal is amplifiedand applied to a voltage controlled integrated circuit oscillator U3 togenerate the signal for switching the audio range tone in bursts in themanner which signals the metal type, as described above. Amplifier U4 bsets the minimum burst rate (pulse rate) at which the voltage controlledoscillator can oscillate to about 2 Hz. Otherwise, a slower rate wouldswitch the audio on and off for periods longer than two seconds, makingit difficult for the user to perceive variations in the pulse rate orgiving the appearance that the sound has stopped. The output ofamplifier U4 b feeds an inhibit input on the voltage controlledoscillator U3, holding it off until an acceptable rate signal isachieved. The switch SW4 e in series with resistor R66 connected toamplifier U4 c provides the user with an option to select a reducedsensitivity. Consequently, the switch SW4 e is mounted to be accessibleon the control panel of an embodiment of the present invention. Thisswitch operates in conjunction with the switch in block 214, describedbelow.

Block 206 is a protection circuit which protects the circuit frombackwardly installed batteries. It turns on to provide a very lowresistance MOSFET 220 pathway only when its gate is positive.

Block 207 is the range signal sample timer corresponding to timingcircuit 104 of FIG. 9. Circuits U8b and Usc set the sample timing forthe range sample after the termination of the current pulse; that is, attime T4 in FIG. 8.

Block 208 has a voltage-controlled oscillator U7, which is controlled bythe magnitude of the range sample. The audio output tone from thevoltage controlled oscillator U7 is applied to an audio amplifier U9 andfrom it to a speaker SPKR1. The inhibit input to the voltage controlledoscillator U7 is driven on and off by the output of the metal-typeidentification voltage control oscillator U3, so that the switching ofthe audio tone to provide the bursts is accomplished by transistorswitching circuitry within the voltage controlled oscillator U7.

Block 209 has a switching regulator U11 with a voltage sense input onits pin 9 (FB). Diode D2 and inductor L2 step up a three volt battery toplus 7 volts, while diodes D2 and D11 take a switcher pulse throughcapacitor C30 and generate a minus 7 volts across capacitor C31. Op-ampU1 b senses the negative supply and provides a regulating input to theswitcher circuit U11.

Block 210 is the astable multivibrator operating to generate the pulsesfor pulsing the coil and corresponds to astable multivibrator 90 in FIG.9. The integrated circuit U13 forming the astable multivibratorgenerates pulses at 240 Hz, as described above. Each pulse therefore hasa duration of 160 microseconds.

Block 211 includes integrated circuit U12 and corresponds to thedivide-by-two, flip-flop 92 described in connection with FIG. 9. Theoutput of the divide-bytwo circuit U12 is fed to a clamping MOSFET Q3.The MOSFET Q3 holds the gate of the output MOSFET Q2 at ground potentialevery other timing cycle. This provides am operating cycle thatalternates between a sample-representing eddy current from a target anda sample representing no eddy current. This difference is used in block216 to eliminate the effects of DC drift in the low-level amplifier U14,as described above. Block 212 provides a resistor network which adds asmall component of the square wave output of integrated circuit U12 tothe low level input to integrated circuit U14. This bias assures thatthe approach to a target always results in an increasing signal, insteadof a slight decrease followed by a large increase. This is because theresidual, no-eddy-current signal can be a slight residual over thetarget-signal that the absolute value detector (U4 b and U4 a in block216) will see in error as a positive target signal.

In block 213, the inductor L1 is the search inductor and corresponds tothe inductor 96 of FIG. 9. It is the main coil. Resistor R65 andpotentiometer R41 provide the resistive damper, described above, in avariable or adjustable manner. Diodes D5 and D6 provide the voltageclamp for the high voltage of the coil spike for protecting the circuitelements, as described above.

Block 214 is a low-level amplifier using op-amp U14. It is necessarythat it have an extremely fast response time in order to recover fromthe overload caused by the voltage spikes which appear on the coil.switch SW4 d provides a lower gain setting and operates in conjunctionwith switch SW4 a in block 205.

Integrated circuit U10 is the high speed-sampling switch describedabove, although only two of its four switching sections are used.

Block 216 uses integrated circuit U6 a to buffer the range sample storedon range sampling capacitor C32. The range signal is a square wave atabout 120 Hz as a result of the on/off action of the divide-by-twocircuit in block 211. The difference between the high and low squarewave levels is a function of the signal strength from a metal target.The DC component is still present on pin 1 of integrated circuit U6 a,but it is removed after passing through capacitor C29. Integratedcircuits U4 b and U4 a detect the absolute value of the range signal,while using diode D7 and D8 to provide a nonlinear gain adjustment tocompensate for a signal that increases by a factor of 64 whenever thedistance to the target is halved; that is, the signal is inverselyproportional to the target distance raised to the power of 6.

Block 217 provides a regulated minus 5 volts from the minus 7 volt powersupply.

FIG. 11 also illustrates the circuitry for connecting a headset to ametal detection circuit of the present invention or alternatively to aconventional second metal detector, so that the user can use both theconventional metal detector and the embodiment of the invention withoutchanging headsets.

A double-throw switch SW3 a,b, which is preferably a double-pole switchfor maintaining left and right channels for the above groundconventional metal detector, has input terminals 301 and 303 connectedto a connector 305 for connection to the audio output of a second,conventional metal detector for connecting the audio from theconventional metal detector to a headset connector 307 when the switchSW3 a,b is in the position illustrated in FIG. 11. Alternatively, whenthe switch SW3 a,b is switched to its other position, the headsetconnector 307 is connected to the audio output from amplifier U9 in theblock 208. Switch SW3 a,b operates simultaneously with switch SW3 c,illustrated in FIG. 10, to switch off battery power to the embodiment ofthe invention when the headset is switched into connection with aconventional metal detector.

Temperature Compensation

There is a need to stabilize the metal type identification signal as thetip temperature changes so that the metal identification signal does notchange as a result of temperature change. If there is no temperaturecompensation, the metal type identification signal will change when theprobe is inserted into a soil which is at a different temperature thanthe probe, which has previously reached thermal equilibrium with thesurface air. The metal type identification signal changes withtemperature, in the absence of thermal compensation, principally becauseof coil resistance changes resulting in a change in the energy stored inthe magnetic fields associated with the probe. In addition to the changein stored inductive energy resulting from the change with temperature ofthe coil's resistance, there is a temperature-dependant change in theferrite core's permeability which also results in a change in storedenergy. A change in stored energy causes a change in the detectedsignals, such as a sample magnitude, which in turn causes misleading orconfusing changes to the signals output to the user.

More particularly, in the uncompensated metal detector described above,the metal type identification feature is greatly affected by thetemperature of the sensing inductor. A small (1 deg. C.) change at theprobe tip causes a noticeable change in the audio pulse rate that isused to signal the detected metal type to the user. Since the ambienttemperature in the ground is usually different from air temperature, theuser must wait for the tip temperature to stabilize after inserting itinto the ground. He must then adjust the pulse control to give a slowpulse rate so that he can observe the change that occurs when heapproaches an underground target.

The target's metal type is determined by measuring the decaying voltagethat develops across the probe's inductor at a specific time after acurrent through the inductor is turned off as described above. A nearbytarget influences the energy stored in the probe's inductor byincreasing or decreasing the permeability of the entire flux path andtherefore the inductance of the inductor. However, the stored energy andthis measured voltage is proportionally influenced by the currentflowing through the inductor during the 165 microsecond pulse whichprecedes the decay illustrated in FIG. 8. In the above describedcircuit, this current is proportional to the resistance of the probe'sinductor. This resistance changes with temperature from about 10 ohms atroom temperature to about 9.5 ohms at 0 deg. C.

The preferred circuit described below and shown in the drawings appendedas FIGS. 15 and 16A through 16D compensates for all temperature effects.A constant current circuit shown in FIG. 17 compensates for the changein the resistance, but does not compensate for the shift in the ferritecore's permeability, and some temperature effect is still noticeable.

FIGS. 15 and 16A through 16D include a full schematic and a simplifieddiagram that illustrate this temperature compensating improvement to theinvention. The improved circuitry shown in FIGS. 15 and 16A through 16Dmeasures and precisely balances any change in the metal typeidentification signal caused by the change in the probe inductor'sresistance and the ferrite's permeability due to temperature changes.

Referring to FIG. 15, a 1 ohm resistor R68 is connected in series withthe probe's inductor L1. Approximately 1 amp current flows through thisinductor during the 165 microsecond pulse applied for the pulseinduction detection method. This current develops a voltage across R68proportional to the current through L1 and, because the resistance ofR68 is much less than the resistance of L1, proportional to L1'sresistance. This voltage is sampled at the end of the 165 microsecondpulse, after the current has reached a steady state in the inductor L1.The sampling is performed by a section of switch U10 and the sample isheld in capacitor C41. This voltage sample is buffered by Dev3 billustrated in FIG. 15 which corresponds to DEVb in FIG. 16A.Potentiometer R72 is set to balance the temperature effect.

The theory of operation of the circuit may begin with the observationthat changes in the temperature of the resistive elements in the proberesult in changes in their resistance, and therefore result in changesin the current through those elements. By measuring the current throughthe coil in the probe, we are effectively measuring the temperature withchanges in that current being proportional to changes in temperature.Consequently, the sampled voltage across the resistor R68 representsprobe temperature and changes in that sampled voltage are a linearfunction of temperature changes. These samples may be referred to astemperature samples.

The ground piercing metal detector is used in soils having a temperaturetypically in the range of 32° F. to 100° F. In that temperature range,the thermal errors introduced into the magnitude of the samples takenfor purpose of metal type identification are essentially a linearfunction of temperature over the typically encountered range of metaltype identification sample magnitudes. As a result, the temperaturesamples may be linearly scaled down to a reduced magnitude essentiallyequal to the the magnitude of the thermal error.

A decrease in temperature of the probe decreases the resistance of thecoil but not the resistance of resistor R68 which is in the circuitrylocated in the detection circuit 28 (FIG. 1) located above ground. Thecoil resistance decrease causes an increase in the current through boththe coil and the resistor R68. This current increase results in anincrease in the thermal sample magnitude and an increase in the energyin the coil and the energy coupled into any metal objects. The increasein the total energy causes an increase in the magnitude of the sampletaken for detecting the metal type as described above. The net effect isthat both the thermal sample magnitude and the metal type detectionsample magnitude increase with a temperature decrease. The opposite istrue for temperature increases. Therefore, since all these changes areessentially linear, a scaled down proportion of the thermal sample maybe subtracted from the metal type detection sample to correct the errorintroduced by temperature changes. This subtraction is performed byapplying both the scaled down error sample and the metal type detectionsample to the input terminal 15 of the differential amplifier U4 c inFIG. 16A. The input circuit to terminal 15 is connected to form asumming circuit connected to perform the subtraction. The scaling of thethermal sample is calibrated by adjustment of the potentiometers R70 andR72 which, along with resistor R69, forms a scaling circuit.

In summary, the thermal sample of the steady state voltage across R68 isa temperature signal which is used as an offset against the temperatureinduced error in the metal type detection sample to balance out andeliminate that error. The circuit is, therefore, a current detectorwhich detects the coil current, by detecting a voltage proportional tothat current, and then scales the detected signal and subtracts aproportion of it from the signal derived from the coil during the metaldetection procedure. This recognizes that the current and currentvariations are proportional to temperature and therefore provide atemperature signal which is used to compensate for thermally causedsignal variations. It should be clear that other current detectioncircuits known to those skilled in the art may be used and their outputsscaled and proportionally subtracted from the detected signal forcompensating temperature variations in the probe.

In use, the user monitors the changes in the signal to detect the metaltype. As described above, the user observes the direction of change inthe pulse rate as an indicator of metal type. For example, if the pulserate increases, a ferrous metal is being observed. The pulse rate slowsfor silver. It is the direction of change and not the amount of changewhich is important. The amount of actual change is typically very small.Therefore, the important thing is to balance out the temperature errorbefore the user detects any metal because this is the beginning pointfrom which changes are referenced.

To accomplish the accurate compensation for temperature, thepotentiometer R72 is adjusted to scale the magnitude of the thermalsample so that there is no change in the metal type signal when theprobe temperature is changed in the absence of any coupled metal object,such as by being inserted in ground away from metal objects. This may bedone as a part of the calibration of the instument under laboratoryconditions.

The verify switch changes the sampling point of the identificationcircuitry to enable the circuit to identify mid-conductivity metals inthe manner described above. For example, the sampling points become T2and T3 shown in FIG. 8. Since the Verify switch's “A” sample point is ata different point on the inductors decay curve illustrated in FIG. 8than the “B” sample point, a different temperature compensation gain isrequired for switch position “A” than is required for position “B”. Onesection of the Verify switch adds resistance network R70 and R 69 whenthe switch is in the “A” position to change the scaling factor andeliminates that resistance network when in the “B” position. To adjustfor thermal compensation, first, potentionmeter R72 is adjusted toeliminate any metal type signal change resulting from temperature changewhen the Verify switch is open in the B position. Then potentiometer R70is adjusted to accomplish the same condition when the Verify switch isclosed in the “A” position.

Another improved circuit is shown in FIG. 17 but is considered less ofan improvement than the above circuit. A constant current circuit isconnected in series with the probe's inductor L1 at the voltage side(not the ground side). This circuit compensates for changes due totemperature changes at the tip. FIG. 17 shows the use of a constantcurrent regulator U1 to improve the temperature stability of the groundpiercing metal detector's metal identification feature.

With reference to the circuit shown in FIG. 17, the strength of themagnetic field developed by the sensing inductor L1 is proportional tothe current flowing through L1 when all other factors are constant. Ifthe inductor were driven by a fixed voltage source, the current would bedetermined by the resistance of L1. This resistance changes withtemperature. Therefore, the current would change with temperature. Thisparticular temperature dependence is reduced by driving the inductor L1with a commercially available constant current regulator so that as theresistance of L1 changes with temperature, the current through L1 doesnot change. This method of temperature stabilization does not compensatefor the change with temperature of the inductor's ferrite core'spermeability. Nor does it compensate for the effects of core saturationwhich occur in the invention at the currents used.

As will be apparent to those skilled in the art, the invention may usemicroprocessor technology so that many of the circuits and proceduresmay be performed by the microcontroller and its software. For example,sample and hold circuits may be controlled by a microcontroller, theiroutputs applied to the microcontroller through analog to digitalconverters and the scaling and mathematical processing may then beperformed by the microcontroller.

While certain preferred embodiments of the present invention have beendisclosed in detail, it is to be understood that various modificationsmay be adopted without departing from the spirit of the invention orscope of the following claims.

What is claimed is:
 1. A hand tool for detecting buried objects, thetool having a hand grip attached to a ground piercing probe which ismanually insertable into soil to displace the soil radially outwardlyfrom the probe, the tool comprising: (a) at least one inductorpositioned in a chamber formed in the probe and connected to a pulseinduction metal detection circuit including a source of inductorcurrent; (b) a current detecting and scaling circuit connected to theinductor to detect a signal representing the current through theinductor; and (c) a summing circuit connected to the output of thescaling circuit for subtracting the current signal from a metaldetection signal derived from the coil by the metal detection circuit toprovide a temperature compensated metal detection signal.
 2. A tool inaccordance with claim 1 wherein a resistor is connected in series withthe inductor and the current detecting and scaling circuit is a sampleand hold circuit having its input connected between the inductor and theresistor.
 3. A tool in accordance with claim 2 wherein the sample andhold circuit includes a resistor network having at least one resistorwhich is alternately switchable into and out of the circuit forproviding alternate scaling factors.
 4. A method for compensating fortemperature variation in signals generated by an inductor mounted in aprobe of a ground piercing metal detector and connected to a metaldetector circuit, the metal detector circuit including a source ofinductor current, to apply metal detection signals to the metal detectorcircuit, the method comprising: (a) detecting a signal representing thecurrent through the inductor by the pulse induction method includingapplication of a pulse to the inductor and wherein the current throughthe inductor is detected by sampling and holding a voltage which isproportional to the inductor current at a time interval near the end ofthe pulse; and (b) subtracting a scaled portion of the current signalfrom the metal detection signal, said scaled portion representingthermal error, to provide a temperature compensated metal detectionsignal.
 5. A metal detection circuit for detecting the metal type of ametal target object, the circuit comprising: (a) a pulse generator; (b)a coil connected to the pulse generator for generating a time changingmagnetic field in response to electrical pulses applied to the coil; (c)a resistive energy damper coupled to the coil for attenuating the energyin the magnetic field; (d) a first sampling and storing circuitconnected to the coil for sampling a signal representing the magnitudeof the coil current at a sampling time within a time interval beginningafter termination of a pulse and ending before the time at whichinductor current in the presence of a high conductivity metal targetobject decays to a current which exceeds the current to which theinductor current decays in the absence of a metal target object; (e) atemperature compensation circuit including a second sampling and storingcircuit connected to the coil for sampling a signal representing coilcurrent at a time interval during the pulse, a scaling circuit and asumming circuit for subtracting a scaled portion of the coil currentsample from the sample obtained by the first sampling and storingcircuit to provide a temperature compensated metal detection signal; and(f) a signaling circuit connected to the first sampling and storingcircuit for signaling changes in the detected coil current at thesampling time as the coil is moved, the signaled changes representingthe metal type of the metal target object.
 6. A method for detecting themetal type of a metal target object buried below the surface of a soil,the method comprising: (a) attempting to induce eddy currents in thetarget object by applying a pulse to an inductor spaced from the targetobject to generate a time changing magnetic field about the inductor;(b) detecting a signal representing the current through the inductornear the end of the pulse; (c) detecting a signal which is a function ofcoil apparent inductance resulting from mutual coupling with the eddycurrents; (d) compensating for temperature variation in the inductor bysubtracting a scaled portion of the current signal from the apparentinductance signal; and (e) signaling the direction of change in theapparent inductance of the inductor as the probe is moved.